Saturday, October 31, 2009

On April 26, 1986, at 1:23 a.m. local time, reactor number four at the Chernobyl Nuclear Power Plant in the Soviet Union exploded. Additional explosions and the resulting fire sent a plume of radioactive fallout into the atmosphere. The fallout released was four hundred times more than had been released by the atomic bombing of Hiroshima.

The plume drifted over extensive parts of:

Western Soviet Union

Western Europe

Eastern Europe

Northern Europe

Eastern North America

Large areas of Russia, Belarus, and the Ukraine were badly contaminated. The contamination resulted in the evacuation of over 300,000 people. About 60% of the radioactive fallout landed in Belarus.

It was the worst nuclear power plant disaster ever, resulting in a severe release of radioactivity into the environment. Two people died in the initial steam explosion, but most deaths from the accident were attributed to radiation.

Russia, the Ukraine, and Belarus are still dealing with the continuing decontamination and health care costs of the Chernobyl accident.

The cost of the disaster is estimated to be around $200 billion USD, making Chernobyl the costliest disaster in modern history.

According to the International Atomic Energy Agency (IAEA) and the World Health Organization (WHO), 56 direct deaths and an estimated 4,000 extra cancer deaths have been attributed to the disaster.

The Chernobyl Exclusion Zone and other limited areas remain off limits. The majority of affected areas are now considered safe for economic activity and settlement.

The Accident

The accident occurred on April 26, 1986, when reactor four suffered a massive, catastrophic power excursion. This resulted in a steam explosion, tearing the top from the reactor, exposing the core, and dispersing large amounts of radioactive particulate and gaseous debris (mostly Strontium-90 and Cesium-137). This allowed oxygen to contact the hot core, which contained 1,700 tons of combustible graphite moderator. The burning graphite moderator increased the emission of the radioactive particles.

The radioactivity was not contained by any type of containment vessel, and radioactive particles were carried by the wind across international borders.

Crisis Management

Radiation Levels

The radiation levels in the worst-hit areas of the reactor building are estimated to have been 5.6 rontgen per second (R/s), or 20,000 R/hr. A lethal dose is around 500 rontgen over 5 hours; some workers received fatal doses within several minutes.

Due to faulty dosimeters (equipment to measure rontgens) or dosimeters which only read low levels of rontogens, the reactor crew chief assumed that the reactor was intact. Operating under this assumption, the chief and his crew stayed in the reactor building until morning trying to pump water into the reactor.

None wore protective gear, and most died from radiation exposure within three weeks.

Fire Containment

Shortly after the accident, firefighters arrived to try to extinguish the fire. They were not told how dangerous the smoke and debris were. They were not told that the fire involved the reactor.

The fires were extinguished by 5 a.m., but many firefighters received high doses of radiation. The fire inside reactor no. 4 continued to burn until May 10, 1986. It was finally extinguished by dropping tons of sand, lead, and clay onto the burning reactor and injecting liquid nitrogen.

Causes of the Disaster

There were two official explanations of the accident:

Flawed operators explanation
Placed the blame on the power plant operators

Flawed design explanation
Placed the blame on flaws in the reactor design, especially the control rods

Effects of the Disaster

International spread of radioactivity

Radioactive release

Human cost
237 people suffered from acute radiation sickness
31 died within the first 3 months after the disaster
135,000 evacuated from the area

Environmental costs
Radioactive contamination of aquatic systems
Four square kilometers of pine forest in the immediate vicinity of the reactor turned brown and died
Some animals in the worst-hit areas died or stopped reproducing

Chernobyl After the Disaster

All work on the unfinished reactors at Chernobyl halted in 1989. A fire broke out in reactor 2 in 1991, resulting in it being taken off-line. Reactor 1 was decommissioned in 1996. Reactor 3 was turned off in 2000, effectively shutting down the entire plant.

Disaster's Effect on Human Health

·57 direct deaths in the accident itself

·4,000 additional cancer cases due to the accident

·Primarily thyroid cancer

·No increase in the rate of birth defects or abnormalities

·No increase in solid cancers

·Possibility of tens of thousands of cases of thyroid cancer in the future.

Mission

This blog is purposely built to give better understanding on the power plant technology especially instrumentation and power plant process control working environment. The writer will update and use this blog as learning process platform and will be sharing among readers.